Partial lift training is one of the least understood aspects of fitness in a lot of communities. This style of training is aptly named because it involves lifting a weight through only a portion of your muscle's range of motion. Partials are generally performed in the strongest range of a lift, for reasons I'll come back to in a bit. A big part of the misconceptions surrounding the effectiveness of partials is that it provides a handicap in your training if you don't perform full range exercises.
The belief is that you will only strengthen the muscle in the range that you train. However, strengthening muscles is truly as simple as increasing the amount of stress on the muscle and having it adapt to that stress accordingly. In addition, a muscle contraction isn't determined by the number of muscle cells that contract but by how many nerves contract. That means that regardless of the muscle position, the entire muscle contracts, so the entire muscle is subject to a significant strength increase. Accordingly, partials in a specific range has the ability to strengthen your muscle in its full range of motion. This makes partials a great addition to any sports regimen, especially if an injury is involved.
And this is another great aspect of partial lift training; partial lifts are great for rehabilitation in regard to a muscle tear, ligament injury, or so forth. Moving a limb through a full range of weighted motion puts too much stress on the injury and dangerously stretches the injury during the motion. This can lead to improper healing, or causing a more potentially devastating injury. Partials can be trained even with an injury because it doesn't put undue stress on the muscles, tendons, or ligaments; the shorter range allows you to improve the blood flow in that area by working it out without making the injury worse. This method of rehabilitation can even make you stronger than you were before your injury.
This leads me to my third appraisal of partials. These lifts can make you much stronger than you were before training them. For instance, try performing a 400 lb leg press from the start of your full range. Depending on your fitness level, this could have varying difficulty, but let's assume that you are able to do so successfully. Now, try leg pressing the same weight in your strongest range; in this instance, it would be 2-5 inches before you completely lock out your knees. You'll find it's almost not fair how much easier it is to move the weight. In fact, you may not even feel as though that's not a sufficient workout. Yet, the fact that the weight is much easier to move there means that it is most definitely a sufficient workout.
This is because you can move much heavier weight in this range. Since you are much stronger in this short range, you can move much heavier amounts of weight than you could hope to imagine in a full range movement. The short range of the movement and the heavy amounts of weight can strengthen the tendons much more than full range of motion training could achieve. You will feel empowered beyond all reason with partial lift training. Your tendons will improve their strength. Your muscles and nervous system will gain true power.
Adding partial lifts to your training regimen will provide you with an unprecedented level of strength. HOWEVER, be very cautious and use all available safety precautions when preparing to perform partial lifts. The exercise is generally much safer than full ROM training, but when dealing with incredible amounts of weight (you may find yourself training in the quadruple digit range), there is always a risk of injury. Be responsible with your training. Don't attempt to leg press 1000 lbs on your first partial lift. The results of this could be anything from lightheartedly laughable to utterly disastrous. Train yourself up gradually; it won't be long before you can manage that amount of weight without undue physical strain. As the old adage goes, “Rome wasn't built in a day.” Your gains in strength and muscle will only occur when you properly rest. The results will be worth it.